Is it possible to have smooth shading on only for rendering?
We can create an operator that will set the shading for all objects. This can be added to the render panel so you have buttons to set smooth and flat shading right above the render button. I have created an addon here to do this, which you can download and install. If you enable the addon and click Save User Settings you can have it enabled every time you run blender.

Keyboard shortcuts can also be setup for quick access, in the addon I setup &#9095; AltF12 to set smooth shading and &#8679; Shift&#9095; AltF12 to set flat shading.